Cell Biology

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Chapter Three
Cell Structures and Their Functions
active transport Carrier-mediated process that requires ATP and can move substances against a concentration gradient. cell membrane [plasma (plaz′m˘ ) membrane] a Outermost component of the cell, surrounding and binding the rest of the cell contents. diffusion (di-f¯ ′zh˘ n) [L. diffundo, to pour u u in different directions] Tendency for solute molecules to move from an area of higher concentration to an area of lower concentration in a solution. endoplasmic reticulum (ER) (en′d¯ o-plas′mik re-tik′¯ um) u-l˘ [endo Gr. plastos, formed] Membranous network inside the cytoplasm; rough ER has ribosomes attached to the surface; smooth ER does not. facilitated diffusion (fa-sil′i-t¯ ˘d di-f¯ ′zh˘ Carrier˘ a-tı u un) mediated process that does not require ATP and moves substances into or out of cells from a higher to a lower concentration. Golgi apparatus (g¯ l′j¯ Stacks of flattened, o e) membrane-bound sacks that collect, modify, package, and distribute proteins and lipids. meiosis (m¯-o′sis)[Gr., a lessening] Process ı ¯ of cell division that results in gametes. Consists of two cell divisions that result in four cells, each of which contains half the number of chromosomes as the parent cell. mitochondrion, pl. mitochondria (m¯′t¯ -kon′dr¯ -on, m¯′t¯ -kon′dre-˘ ) ı o e ı o ¯ a [Gr. mitos, thread chandros, granule] Small, bean-shaped or rod-shaped structures in the cytoplasm that are sites of ATP production. mitosis (m¯-t¯ ı o′sis) [Gr., thread] Division of the nucleus. Process of cell division that results in two daughter cells with exactly the same number and type of chromosomes as the parent cell. nucleus, pl. nuclei (noo′kl¯ -˘ s, noo′kl¯ -¯) [L., inside of eu e ı a thing] Cell organelle containing most of the cell’s genetic material. osmosis (os-m¯ o′sis) [Gr. osmos, thrusting or an impulsion] Diffusion of solvent (water) through a selectively permeable membrane from a region of higher water concentration to one of lower water concentration. ribosome (r¯′bo-som) Small, spherical, ı ¯ ¯ cytoplasmic organelle where protein synthesis occurs.

Objectives
After reading this chapter, you should be able to: 1. Describe the structure of the cell membrane. 2. Describe the structure and function of the nucleus and nucleoli. 3. Compare the structure and function of rough and smooth endoplasmic reticulum. 4. Describe the roles of the Golgi apparatuses and secretory vesicles in secretion. 5. Explain the role of lysosomes in digesting material taken into cells by phagocytosis. 6. Describe the structure and function of mitochondria. 7. Compare the structure and function of cilia, flagella, and microvilli. 8. List four ways by which substances cross the cell membrane. 9. Explain the role of osmosis and that of osmotic pressure in controlling the movement of water across the cell membrane. Compare hypotonic, isotonic, and hypertonic solutions. 10. Define “mediated transport,” and compare the processes of facilitated diffusion, active transport, and secondary active transport. 11. Describe endocytosis and exocytosis. 12. Describe the process of protein synthesis. 13. Explain what is accomplished during mitosis and meiosis. 14. Define “differentiation,” and explain how it occurs.

The cell is the basic living unit of all organisms. The simplest organisms consist of a single cell, whereas humans are composed of trillions of cells. If each of these cells was about the size of a standard brick, we could build a colossal structure in the shape of a human over 5 1⁄2 miles (10 km) high! Obviously, there are many differences between a cell and a brick. Cells are much smaller than bricks: An average-sized cell is one fifth the size of the smallest dot you can make on a sheet of paper with a sharp pencil! In spite of their extremely small size, cells are complex living structures. Cells of the human body have many characteristics in common. However, most cells are also specialized to perform specific functions. The human body is made up of populations of these specialized cells. Communication and coordination between these populations are critical for a complex organism, such as a human, to survive. The study of cells is an important link between the study of chemistry in chapter 2 and tissues in chapter 4. A knowledge of chemistry makes it possible to understand cells because cells are composed of molecules that are responsible for many of the characteristics of cells. Cells, in turn, determine the form and functions of the tissues of the body. It is also important to understand that a great many diseases and other human disorders have a cellular basis. This chapter considers the structure of cells and how cells perform the activities necessary for life.

Cell Structure
Each cell is a highly organized unit. Within cells, specialized structures called organelles (or′g˘ -nelz, “little organs”) pera form specific functions (figure 3.1 and table 3.1). The nucleus is an organelle containing the cell’s genetic material. The living material surrounding the nucleus is called cytoplasm (s¯ o-plazm), which contains many other types of organelles. ı′t¯ The cytoplasm is enclosed by the cell, or plasma, membrane. The number and type of organelles within each cell determine the cell’s specific structure and functions. For example, cells secreting large amounts of protein contain welldeveloped organelles that synthesize and secrete protein, whereas muscle cells have organelles that enable the cells to contract. The following sections describe the structure and main functions of the major organelles found in cells.

Cell Membrane
The cell membrane, or plasma (plaz′m a) membrane, is the ˘ outermost component of a cell. The cell membrane encloses the cytoplasm and forms the boundary between material inside the cell and material outside it. Substances outside the cell are called extracellular substances, and substances inside the cell are called intracellular substances. The cell membrane encloses the cell, supports the cell contents, is a selective barrier that determines what moves into and out of the cell, and plays a role in communication between cells. The major molecules that make up the cell membrane are phospholipids and proteins. In addition, the membrane contains other molecules, such as cholesterol, carbohydrates, water, and ions. The phospholipids form a double layer of molecules. The polar, phosphate-containing ends of the phospholipids are hydrophilic (water loving) and therefore face the water inside and outside the cell. The nonpolar, fatty acid ends of the phospholipids are hydrophobic (water fearing) and therefore face away from the water on either side of the membrane, toward the center of the double layer of phospholipids (figure 3.2). The double layer of phospholipids forms a lipid barrier between the inside and outside of the cell. Studies of the arrangement of molecules in the cell membrane have given rise to a model of its structure called the fluid mosaic model. The double layer of phospholipid molecules has a liquid quality. Cholesterol within the membrane gives it added strength and flexibility. Protein molecules “float” among the phospholipid molecules and, in some cases, may extend from the inner to the outer surface of the cell membrane. Carbohydrates may be bound to some protein molecules, modifying their functions. The proteins function as membrane channels, carrier molecules, receptor molecules, enzymes, or structural supports in the membrane. Membrane channels and carrier molecules are involved with the movement of substances through the cell membrane. Receptor molecules are part of an intercellular communication system that enables coordination of the activities of cells. For example, a nerve cell can release a chemical messenger that moves to a muscle cell and temporarily binds to its receptor. The binding acts as a signal that triggers a response such as contraction of the muscle cell.

Functions of the Cell
The main functions of the cell include 1. Basic unit of life. The cell is the smallest part to which an organism can be reduced that still retains the characteristics of life. 2. Protection and support. Cells produce and secrete various molecules that provide protection and support of the body. For example, bone cells are surrounded by a mineralized material, making bone a hard tissue that protects the brain and other organs and that supports the weight of the body. 3. Movement. All the movements of the body occur because of molecules located within specific cells such as muscle cells. 4. Communication. Cells produce and receive chemical and electrical signals that allow them to communicate with one another. For example, nerve cells communicate with one another and with muscle cells, causing them to contract. 5. Cell metabolism and energy release. The chemical reactions that occur within cells are referred to collectively as cell metabolism. Energy released during metabolism is used for cell activities, such as the synthesis of new molecules, muscle contraction, and heat production, which helps maintain body temperature. 6. Inheritance. Each cell contains a copy of the genetic information of the individual. Specialized cells are responsible for transmitting that genetic information to the next generation.

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Chapter Three

Cell Structures and Their Functions

Figure 3.1 Generalized Cell Showing the Major Organelles
No single cell contains all organelle types. In addition, some kinds of cells contain many organelles of one type, and another kind of cell contains very few.

Table 3.1
Organelles
Nucleus

Organelles and Their Locations and Functions
Location and Function(s)
Usually near center of the cell; contains genetic material of cell (DNA) and nucleoli; site of ribosome and messenger RNA synthesis In the nucleus; site of ribosomal RNA and ribosomal protein synthesis In cytoplasm; many ribosomes attached to ER; site of protein synthesis In cytoplasm; site of lipid synthesis In cytoplasm; modifies protein structure and packages proteins in secretory vesicles In cytoplasm; contains materials produced in the cell; formed by the Golgi apparatus; secreted by exocytosis In cytoplasm; contains enzymes that digest material taken into the cell In cytoplasm; site of aerobic respiration and the major site of ATP synthesis In cytoplasm; supports cytoplasm; assists in cell division and forms components of cilia and flagella On cell surface with many on each cell; cilia move substances over surface of certain cells On sperm cell surface with one per cell; propels the sperm cells Extensions of cell surface with many on each cell; increase surface area of certain cells

Nucleolus Rough endoplasmic reticulum (rough ER) Smooth endoplasmic reticulum (smooth ER) Golgi apparatus Secretory vesicle Lysosome Mitochondrion Microtubule Cilia Flagella Microvilli

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Cell Structure

Figure 3.2 The Cell Membrane
The cell membrane is composed of a double layer of phospholipid molecules with proteins “floating” in the membrane. The nonpolar end of each phospholipid molecule is directed toward the center of the membrane, and the polar end of each phospholipid molecule is directed toward the water environment either outside or inside the cell. Cholesterol molecules are interspersed among the phospholipid molecules. Groups of proteins can form membrane channels, carrier molecules, receptor molecules, enzymes, or structural supports.

Nucleus
The nucleus (noo′kl¯ -˘ s) is a large organelle usually located eu near the center of the cell (see figure 3.1). All cells of the body have a nucleus at some point in their life cycle, although some cells, such as red blood cells, lose their nuclei as they mature. Other cells, such as osteoclasts (a type of bone cell) and skeletal muscle cells, contain more than one nucleus. The nucleus is bounded by a nuclear envelope, which consists of outer and inner membranes with a narrow space between them (figure 3.3). At many points on the surface of the nucleus, the inner and outer membranes come together to form nuclear pores, through which materials can pass into or out of the nucleus. The nucleus contains loosely coiled fibers called chromatin consisting of deoxyribonucleic acid (DNA) and proteins (see figures 2.17 and 3.3b). During cell division, the chromatin fibers become more tightly coiled to form the 23 pairs of chromosomes (kro′m¯ -s¯ mz) characteristic of human cells (see the ¯ o o section on Cell Division on p. 59). The genes that influence the structural and functional features of every individual are made up of DNA molecules. The DNA molecules store information that allows the genes to determine the structure of proteins.

surrounding membrane (see figure 3.3). The subunits of ribosomes are formed within a nucleolus. Proteins produced in the cytoplasm move through the nuclear pores into the nucleus and to the nucleolus. These proteins are joined to ribosomal ribonucleic (r¯′b¯ -noo-kl¯ ′ik) acid (rRNA), proı o e duced within the nucleolus, to form large and small ribosomal subunits (figure 3.4). The ribosomal subunits then move from the nucleus through the nuclear pores into the cytoplasm, where one large and one small subunit join to form a ribosome. Ribosomes (r¯′b¯ -s¯ mz) are the organelles where proı o o teins are produced (see section on Protein Synthesis on p. 56). Free ribosomes are not attached to any other organelles in the cytoplasm, whereas other ribosomes are attached to a membrane called the endoplasmic reticulum.

Rough and Smooth Endoplasmic Reticulum
The endoplasmic reticulum (en′d¯ -plas′mik re-tik′¯ -l˘ m) (ER) o u u is a series of membranes that extends from the outer nuclear membrane into the cytoplasm (figure 3.5). Rough ER is ER with ribosomes attached to it. A large amount of rough ER in a cell indicates that it is synthesizing large amounts of protein for export from the cell. On the other hand, ER without ribosomes is called smooth ER. Smooth ER is a site for lipid synthesis in cells. Smooth ER also participates in detoxification of chemicals within the cell. In skeletal muscle cells, the smooth ER stores calcium ions.

Nucleoli and Ribosomes
Nucleoli (n oo-kl¯ ′¯ -l¯) number from one to four per nucleus. eo ı They are rounded, dense, well-defined nuclear bodies with no 44

Chapter Three

Cell Structures and Their Functions

(a)

(b)

(c)

Figure 3.3 The Nucleus
(a) The nuclear envelope consists of inner and outer membranes that become fused at the nuclear pores. The nucleolus is a condensed region of the nucleus not bounded by a membrane and consisting mostly of RNA and protein. (b) Transmission electron micrograph of the nucleus. (c) Scanning electron micrograph showing the inner surface of the nuclear envelope and the nuclear pores.

The Golgi Apparatus
The Golgi (gol′j¯ ) apparatus (named for Camillo Golgi e [1843–1926], an Italian histologist) consists of closely packed stacks of curved, membrane-bound sacs (figure 3.6). It collects, modifies, packages, and distributes proteins and lipids manufactured by the ER. For example, proteins produced at the ribosomes enter the Golgi apparatus from the ER. In some cases, the Golgi apparatus chemically modifies the proteins by attaching carbohydrate or lipid molecules to them. The proteins then are packaged into membrane sacs that pinch off from the margins of the Golgi apparatus (see section on Secretory Vesicles below). The Golgi apparatus is present in larger numbers and is most highly developed in cells that secrete protein, such as the cells of the salivary glands or the pancreas.

cell (see figure 3.6). Their membranes then fuse with the cell membrane, and the contents of the vesicles are released to the exterior of the cell. In many cells, secretory vesicles accumulate in the cytoplasm and are released to the exterior when the cell receives a signal. For example, secretory vesicles containing the hormone insulin remain in the cytoplasm of pancreatic cells until rising blood levels of glucose act as a stimulus for their release.

Lysosomes
Lysosomes (l¯′s¯ -s¯ mz) (see figure 3.1) are membrane-bound ı o o vesicles formed from the Golgi apparatus. They contain a variety of enzymes that function as intracellular digestive systems. Particulate material taken into a cell is contained within vesicles that fuse with lysosomes. The enzymes within the lysosomes break down the ingested materials. For example, white blood cells take up bacteria, which the enzymes within lysosomes destroy. Also, when tissues are damaged, ruptured lysosomes within the damaged cells release their enzymes and digest both healthy and damaged cells. The released enzymes are responsible for part of the resulting inflammation (see chapter 4). 45

Secretory Vesicles
A vesicle (ves′i-kl) is a small, membrane-bound sac that transports or stores materials within cells. Secretory vesicles pinch off from the Golgi apparatus and move to the surface of the

Cell Structure

Figure 3.4 Production of Ribosomes

Did You Know?
Some diseases result from nonfunctional lysosomal enzymes. For example, Pompe’s disease results from the inability of lysosomal enzymes to break down the carbohydrate glycogen produced in certain cells. Glycogen accumulates in large amounts in the heart, liver, and skeletal muscles. Glycogen accumulation in the heart muscle cells often leads to heart failure. Lipid storage disorders are often hereditary and are characterized by the accumulation of large amounts of lipid in phagocytic cells. These cells take up the lipid by phagocytosis, but they lack the enzymes required to break down the lipid droplets. Symptoms include enlargement of the spleen and liver and replacement of bone marrow by lipid-filled phagocytes.

Peroxisomes
Figure 3.5 The Endoplasmic Reticulum
The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (ER). Rough ER has ribosomes attached to its membrane, and smooth ER has no ribosomes attached to it. Some cells contain predominantly smooth ER, and others contain predominantly rough ER.

Peroxisomes (per-ok′si-somz) are small, membrane-bound ¯ vesicles containing enzymes that break down fatty acids and amino acids. Hydrogen peroxide (H 2O2), which can be toxic to the cell, is a by-product of that breakdown. Peroxisomes also contain an enzyme that breaks down hydrogen peroxide to water and oxygen. Cells that are active in detoxification, such as liver and kidney cells, have many peroxisomes.

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Chapter Three

Cell Structures and Their Functions

Mitochondria
Mitochondria (m¯′t¯ -kon′dr e- a; sing. mitochondrion) are ı o ¯ ˘ small, bean-shaped or rod-shaped organelles with inner and outer membranes separated by a space (figure 3.7 and see figure 3.1). The outer membranes have a smooth contour, but the inner membranes have numerous infoldings called cristae (kris′t¯ ), which project like shelves into the interior of the e mitochondria. Mitochondria are the major sites of adenosine triphosphate (ATP) production within cells. ATP is the major energy source for most chemical reactions within the cell, and cells with a large energy requirement have more mitochondria than cells that require less energy. Mitochondria carry out aerobic respiration (discussed in greater detail in the section Cell Metabolism on p. 54) in which oxygen is required to allow the reactions that produce ATP to proceed. Cells that carry out extensive active transport, which is described on p. 54, contain many mitochondria, and, when muscles enlarge as a result of exercise, the mitochondria increase in number within the muscle cells and provide the additional ATP required for muscle contraction. Increases in the number of mitochondria result from the division of preexisting mitochondria. The information for making some mitochondrial proteins and for mitochondrial division is contained in a unique type of DNA within the mitochondria. This DNA is more like bacterial DNA than that of the cell’s nucleus.

Cytoskeleton
The cytoskeleton (s¯ o-skel′˘ -ton) consists of proteins that ı-t¯ e support the cell, hold organelles in place, and enable the cell to change shape. The cytoskeleton consists of microtubules, microfilaments, and intermediate filaments (figure 3.8). Microtubules are hollow structures formed from protein subunits that perform a variety of roles, such as helping to provide support to the cytoplasm of cells, assisting in the process of cell division, and forming essential components of certain organelles such as cilia and flagella. Microfilaments are small fibrils formed from protein subunits that structurally support the cytoplasm. Some microfilaments are involved with cell movements. For example, microfilaments in muscle cells enable the cells to shorten or contract. Intermediate filaments are fibrils formed from protein subunits that are smaller in diameter than microtubules but larger in diameter than microfilaments. They provide mechanical support to the cell.

Cilia, Flagella, and Microvilli
Cilia (s˘ l′¯ -˘ ) project from the surface of cells, are capable of ı ea moving (see figure 3.1), and vary in number from none to thousands per cell. Cilia have a cylindrical shape, contain specialized microtubules, and are enclosed by the cell membrane. Cilia are numerous on surface cells that line the respiratory tract. Their coordinated movement moves mucus, in which

Figure 3.6 The Golgi Apparatus

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Movement Through the Cell Membrane

(a)

(b)

Figure 3.7 Mitochondria
(a) Typical mitochondrion structure. (b) Electron micrograph of mitochondria in longitudinal and cross sections.

dust particles are embedded, upward and away from the lungs. This action helps keep the lungs clear of debris. Flagella (fl˘ -jel′˘ ) have a structure similar to that of cilia a a but are much longer, and usually occur only one per cell. Sperm cells each have one flagellum, which functions to propel the sperm cell. Microvilli (m¯′kr¯ -vil′¯) are specialized extensions of the ı o ı cell membrane that are supported by microfilaments (see figure 3.1), but they do not actively move like cilia and flagella. Microvilli are numerous on cells that have them and function to increase the surface area of those cells. They are abundant on the surface of cells that line the intestine, kidney, and other areas in which absorption is an important function.

Movement Through the Cell Membrane
The cell membrane is selectively permea ble, allowing some substances, but not others, to pass into or out of the cell. Intracellular material has a different composition from extracellular material, and the survival of cells depends on maintaining the difference. Substances such as enzymes, glycogen, and potassium ions are found at higher concentrations intracellularly; and sodium, calcium, and chloride ions are found in greater concentrations extracellularly. In addition, nutrients must enter cells continually, and waste products must exit. Because of the permeability characteristics of the cell membrane and its ability to transport certain molecules, cells are able to maintain proper intracellular concentrations of molecules. Rupture of the membrane, alteration of its permeability characteristics, or inhibition of transport processes can disrupt the normal intracellular concentration of molecules and lead to cell death. Molecules can pass through the cell membrane in four ways: 1. Directly through the phospholipid membrane. Molecules that are soluble in lipids, such as oxygen, carbon dioxide, and steroids, pass through the cell membrane readily by dissolving in the lipid bilayer. The phospholipid bilayer acts as a barrier to most substances that are not lipid-soluble; but certain small, non-lipid-soluble molecules, such as water, and urea, can diffuse between the phospholipid molecules of the cell membrane. 2. Membrane channels. Cell membrane channels, consisting of large protein molecules, extend from one surface of the cell membrane to the other (see figure 3.2). There are several channel types, each of which allows only certain molecules to pass through it. The size, shape, and charge of molecules determines whether they can pass through each kind of channel. For example, sodium ions pass through sodium channels, and potassium and chloride ions

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List the organelles that are common in cells that (a) synthesize and secrete proteins, (b) actively transport substances into cells, and (c) ingest foreign substances. Explain the function of each organelle you list. Answer on page 00

Whole-Cell Activity
To understand how a cell functions, the interactions between the organelles must be considered. For example, the transport of many food molecules into the cell requires ATP and cell membrane proteins. Most ATP is produced by mitochondria. The production of cell membrane proteins requires amino acids that are transported into the cell across the cell membrane by transport proteins. Information contained in DNA within the nucleus determines which amino acids are combined at ribosomes to form proteins. The mutual interdependence of cellular organelles is coordinated to maintain homeostasis within the cell and the entire body. The following sections, Movement Through the Cell Membrane, Cell Metabolism, Protein Synthesis, and Cell Division, illustrate the interactions of organelles that result in a functioning cell.

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Chapter Three

Cell Structures and Their Functions

(a)

(b)

Figure 3.8 Cytoskeleton
(a) Microtubules, microfilaments, and intermediate filaments form the cytoskeleton. (b) Scanning electron micrograph of the cytoskeleton.

pass through potassium and chloride channels, respectively. Rapid movement of water across the cell membrane apparently occurs through membrane channels. 3. Carrier molecules. Large polar molecules that are not lipid-soluble, such as glucose and amino acids, cannot pass through the cell membrane in significant amounts unless they are transported by special carrier molecules. Substances that are transported across the cell membrane by carrier molecules are said to be transported by carrier-mediated processes. The carrier molecules are proteins that extend from one side of the cell membrane to the other. They bind to molecules to be transported and move them across the cell membrane. Each carrier molecule transports a specific type of molecule. For example, carrier molecules that transport glucose across the cell membrane do not transport amino acids, and carrier molecules that transport amino acids do not transport glucose. 4. Vesicles. Large non-lipid-soluble molecules, small pieces of matter, and even whole cells can be transported across the cell membrane in a vesicle, which is a membrane-bound sac. Because of the fluid nature of membranes, the vesicle and the cell membrane can fuse, allowing the contents of the vesicle to cross the cell membrane.

Diffusion
A solution is a solid, liquid, or gas and consists of one or more substances called solutes dissolved in the predominant solid, liquid, or gas, which is called the solvent.Diffusion can be viewed as the tendency for solutes, such as ions or molecules, to move from an area of higher concentration to an area of lower concentration in solution (figure 3.9a and b, and table 3.2). Examples of diffusion are the movement and distribution of smoke or perfume throughout a room in which there are no air currents, or that of a dye throughout a beaker of still water. Diffusion is a product of the constant random motion of all solutes in a solution. More solute particles occur in an area of higher concentration than in one of lower concentration. Because particles move randomly, the chances are greater that solute particles will move from the higher toward the lower concentration than from a lower to higher concentration. At equilibrium, the net movement of solutes stops, although the random motion continues, and the movement of solutes in any one direction is balanced by an equal movement in the opposite direction (figure 3.9c). A concentration gradient is a measure of the difference in the concentration of a solute in a solvent between two points. For a given distance between two points, the concentration gradient is equal to the higher concentration minus the lower concentration of a solute. Movement down, or with, a

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Movement Through the Cell Membrane

(a)

(b)

(c)

Figure 3.9 Diffusion

Table 3.2
Type
Diffusion

Types and Characteristics of Movement Across Cell Membranes
Transport
With the concentration gradient through the lipid portion of the cell membrane or through membrane channels With the concentration gradient (for water) through the lipid portion of the cell membrane or through membrane channels Movement of liquid and substances by pressure through a partition containing holes With the concentration gradient by carrier molecules Against the concentration gradient* by carrier molecules Against the concentration gradient by carrier molecules; the energy for secondary active transport of one substance comes from the concentration gradient of another Movement into cells by vesicles Movement out of cells by vesicles

Requires ATP
No

Examples
Oxygen, carbon dioxide, chloride ions, and urea Water

Osmosis

No

Filtration

No

In the kidneys, filtration of everything in blood except proteins and blood cells Glucose in most cells Sodium, potassium, calcium, and hydrogen ions; amino acids Glucose, amino acids

Facilitated diffusion Active transport Secondary active transport

No Yes Yes

Endocytosis Exocytosis

Yes Yes

Ingestion of particles by phagocytosis and liquids by pinocytosis Secretion of proteins

*Active transport normally moves substances against their concentration gradient, but it can also move substances with their concentration gradient.

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Chapter Three

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Clinical Focus

Relationships Between Cell Structure and Cell Function
secretory vesicles, and cilia. The ribosomes on the rough ER are the sites where proteins, a major component of mucus, are produced. The Golgi apparatuses package the proteins and other components of mucus into secretory vesicles, which move to the surface of the epithelial cells. The contents of the secretory vesicles are released onto the surface of the epithelial cells. Cilia on the cell surface then propel the mucus toward the throat. In people who smoke, the prolonged exposure of the respiratory epithelium to the irritation of tobacco smoke causes the respiratory epithelial cells to change in structure and function. The cells flatten and form several layers of epithelial cells. These flattened epithelial cells no longer contain abundant rough ER, Golgi apparatuses, secretory vesicles, or cilia. The respiratory epithelium is adapted to protect the underlying cells from irritation, but once altered by smoking it can no longer function to secrete mucus and transport it toward the throat to clean the respiratory passages. Extensive replacement of normal epithelial cells in respiratory passages is associated with chronic inflammation of the respiratory passages (bronchitis), which is common in people who smoke heavily.

Each cell is well adapted for the functions it performs, and the abundance of organelles in each cell reflects the function of the cell. For example, epithelial cells that line the larger-diameter respiratory passages secrete mucus and transport it toward the throat, where it is either swallowed or expelled from the body by coughing. Particles of dust and other debris suspended in the air become trapped in the mucus. The production and transport of mucus from the respiratory passages function to keep these passages clean. Cells of the respiratory system have abundant rough ER, Golgi apparatuses,

concentration gradient, describes the diffusion of solutes from a higher toward a lower concentration of solutes. Movement up, or against, a concentration gradient, describes the movement of solutes from a lower toward a higher concentration of solutes. This second type of movement does not occur by diffusion and requires energy to move solutes against their concentration gradient. The concentration gradient is said to be steeper when the concentration gradient is large. Diffusion is an important means of transporting substances through the extracellular and intracellular fluids in the body. In addition, substances that can pass either through the lipid layers of the cell membrane or through membrane channels diffuse through the cell membrane. Some nutrients enter and some waste products leave the cell by diffusion. The normal intracellular concentrations of many substances depend on diffusion. For example, if the extracellular concentration of oxygen is reduced, not enough oxygen diffuses into the cell, and normal cell function cannot occur.

Osmosis
Osmosis (os-m¯ ′sis) is the diffusion of water (a solvent) across o a selectively permeable membrane, such as the cell membrane, from a region of higher water concentration to one of lower water concentration (see table 3.2). Osmosis is important to cells because large volume changes caused by water movement can disrupt normal cell functions. Osmosis occurs when the cell membrane is either less permeable or not permeable to solutes and a concentration gradient for water exists across the cell membrane. Water diffuses from a solution with a higher concentration of water across the cell membrane into a solution with a lower water concentration. The ability to predict the direction of water movement depends on knowing which solution on either side of a membrane has the highest water concentration. The concentration of a solution, however, is not expressed in terms of water, but in terms of solute concentration. For example, if sugar solution A is twice as concentrated as sugar solution B, then solution A has twice as much sugar (solute) as solution B. As the concentration of a solution increases, the amount of water (solvent) proportionately decreases. Thus water diffuses from the less concentrated solution, which has fewer solute molecules and more water molecules, into the more concentrated solution with more solute molecules and fewer water molecules.

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Urea is a toxic waste produced inside liver cells. It diffuses from those cells into the blood and is eliminated from the body by the kidneys. What would happen to the intracellular and extracellular concentration of urea if the kidneys stopped functioning? Answer on page 00

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Movement Through the Cell Membrane

(a)

(b)

(c)

Figure 3.10 Osmosis Osmotic pressure is the force required to prevent the movement of water across a selectively permeable membrane. Thus osmotic pressure is a measure of the tendency of water to move by osmosis across a selectively permeable membrane. It can be measured by placing a solution into a tube that is closed at one end by a selectively permeable membrane and immersing the tube in distilled water (figure 3.10a). Water molecules move by osmosis through the membrane into the tube, forcing the solution to move up the tube (figure 3.10b). As the solution rises, its weight produces hydrostatic pressure (figure 3.10c), which moves water out of the tube back into the distilled water surrounding the tube. Net movement of water into the tube stops when the hydrostatic pressure in the tube causes water to move out of the tube at the same rate that it diffuses into the tube by osmosis. The osmotic pressure of the solution in the tube is equal to the hydrostatic pressure that prevents net movement of water into the tube. The greater the concentration of a solution, the greater its osmotic pressure, and the greater the tendency for water to move into the solution. This occurs because water moves from

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Chapter Three

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Figure 3.11 Effects of Hypotonic, Isotonic, and Hypertonic Solutions on Red Blood Cells

less concentrated solutions (less solute, more water) into more concentrated solutions (more solute, less water). The greater the concentration of a solution (the less water it has), the greater the tendency for water to move into the solution, and the greater the osmotic pressure must be to prevent that movement. Cells will either swell, remain unchanged, or shrink when placed into a solution. When a cell is placed into a hypotonic (h¯′p¯ -ton′ik) solution, the solution usually has a ı o lower concentration of solutes and a higher concentration of water than the cytoplasm of the cell. Water moves by osmosis into the cell, causing it to swell. If the cell swells enough, it can rupture, a process called lysis (l¯′sis) (figure 3.11a). When ı a cell is immersed in an isotonic (¯′s¯ -ton′ik) solution, the ı o concentrations of various solutes and water are the same on both sides of the cell membrane. The cell therefore neither shrinks nor swells (figure 3.11b). When a cell is immersed in a hypertonic (h¯′per-ton′ik) solution, the solution usually has ı a higher concentration of solutes and a lower concentration of water than the cytoplasm of the cell. Water moves by osmosis from the cell into the hypertonic solution, resulting in cell shrinkage, or crenation (kr e-n¯ ′sh un) (figure 3.11c). Solutions ¯ a ˘ injected into the circulatory system or into tissues must be isotonic because swelling or shrinking disrupts the normal function of cells and can lead to cell death.

a car, oil but not dirt particles passes through an oil filter. In the body, filtration occurs in the kidneys as a step in urine production. Blood pressure moves fluid from the blood through a partition, or filtration membrane. Water, ions, and small molecules pass through the filtration membrane as a step in urine formation, whereas larger substances, such as proteins and blood cells, remain in the blood (see chapter 18).

Mediated Transport Mechanisms
Many nutrient molecules, such as amino acids and glucose, cannot enter the cell by the process of diffusion, and many substances, such as proteins, produced in cells cannot leave the cell by diffusion. Carrier molecules within the cell membrane are involved in carrier-mediated transport mechanisms, which function to move large, water-soluble molecules or electrically charged ions across the cell membrane. After a molecule to be transported binds to a carrier molecule on one side of the membrane, the three-dimensional shape of the carrier molecule changes, and the transported molecule is moved to the opposite side of the cell membrane (figure 3.12). The transported molecule is then released by the carrier molecule, which resumes its original shape and is available to transport another molecule. There are three kinds of mediated transport: facilitated diffusion, active transport, and secondary active transport.

Filtration
Filtration is the movement of fluid through a partition containing small holes (see table 3.2). The fluid movement results from the pressure or weight of the fluid pushing against the partition. The fluid and substances small enough to pass through the holes move through the partition, but substances larger than the holes do not pass through it. For example, in

Facilitated Diffusion
Facilitated diffusion is a mediated transport process that moves substances into or out of cells from a higher to a lower concentration (see table 3.2). Because movement is with the concentration gradient, metabolic energy in the form of ATP is not required.

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Movement Through the Cell Membrane

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The transport of glucose into most cells occurs by facilitated diffusion. Because diffusion occurs from a higher to a lower concentration, glucose cannot accumulate within these cells at a higher concentration than is found outside the cell. Once glucose enters a cell, it is rapidly converted to other molecules, such as glucose phosphate or glycogen. What effect does this conversion have on the ability of the cell to transport glucose? Answer on page 00

into the cell, down its concentration gradient, provides the energy to transport a different substance, such as glucose, into the cell (figure 3.14).

Endocytosis and Exocytosis
Endocytosis (en′d¯ -s¯ o′sis) is the uptake of material through o ı-t¯ the cell membrane by the formation of a membrane-bound sac called a vesicle (see table 3.2). The two types of endocytosis are phagocytosis and pinocytosis. Phagocytosis (fag′¯ -s¯-t¯ ′sis) means “cell eating” and apo ı o plies to endocytosis when solid particles are ingested. A part of the cell membrane extends around a particle and fuses so that the particle is surrounded by the membrane. That part of the membrane then “pinches off” to form a vesicle containing the particle. The vesicle is within the cytoplasm of the cell, and the cell membrane is left intact (figure 3.15). White blood cells and some other cell types phagocytize bacteria, cell debris, and foreign particles. Phagocytosis is an important means by which white blood cells take up and destroy harmful substances that have entered the body. Pinocytosis (pin′¯ -s¯-t o′sis) means “cell drinking.” It is o ı ¯ distinguished from phagocytosis in that much smaller vesicles are formed, they contain liquid rather than particles, and the cell membrane invaginates to form the vesicles that are taken into the cell. Pinocytosis is a common transport mechanism and occurs in certain kidney cells, epithelial cells of the intestine, liver cells, and cells that line capillaries. In some cells, secretions accumulate within vesicles. These secretory vesicles then move to the cell membrane, where the vesicle membrane fuses with the cell membrane, and the content of the vesicle is eliminated from the cell (see figure 3.6). This process is called exocytosis (ek′s¯ -s¯-t¯ ′sis) o ı o (figure 3.16 and see table 3.2). Secretion of digestive enzymes by the pancreas, of mucus by the salivary glands, and of milk from the mammary glands are examples of exocytosis. In many respects the process is similar to that of endocytosis, but it occurs in an opposite direction. Endocytosis results in the uptake of materials by cells, and exocytosis in the release of materials from cells. Both endocytosis and exocytosis require energy in the form of ATP to form vesicles.

Active Transport
Active transport is a carrier-mediated process that moves substances from regions of lower concentration to ones of higher concentration against a concentration gradient (see table 3.2). Consequently, active transport processes accumulate substances on one side of the cell membrane at concentrations many times greater than those on the other side. Active transport requires energy in the form of ATP, and if ATP is not available, active transport stops. Examples of active transport include the movement of amino acids from the small intestine into the blood. In some cases, the active transport mechanism can exchange one substance for another. For example, the sodium– potassium exchange pump moves sodium ions out of cells and potassium ions into cells (figure 3.13). The result is a higher concentration of sodium ions outside the cell and a higher concentration of potassium ions inside the cell. The concentration gradients for sodium and potassium ions, established by the sodium–potassium exchange pump, are essential in maintaining the resting membrane potential (see chapter 8). Cystic fibrosis is a genetic disorder that affects the active transport of chlorine ions into cells. This disorder is discussed in the Systems Pathology essay on p. 66.

Secondary Active Transport
Secondary active transport involves the active transport of one substance, such as an ion, out of a cell, establishing a concentration gradient. The diffusion of the substance back

Cell Metabolism
Cell metabolism is the sum of all the chemical reactions in the cell (figure 3.17). The breakdown of food molecules releases energy that is used to synthesize ATP (see chapter 17). When ATP is broken down, energy is released which can be used to drive other chemical reactions or processes such as active transport. The breakdown of the sugar glucose, such as the sugar from a candy bar, by a series of reactions within the cytoplasm

Figure 3.12 Mediated Transport Mechanism
(a) A molecule binds to a protein carrier molecule on one side of the cell membrane. (b) The carrier molecule changes shape and releases the molecule on the other side of the cell membrane.

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Figure 3.13

Sodium–Potassium Exchange Pump

of a cell is called glycolysis (gl¯-kol′i-sis). Glucose is conı verted to pyruvic acid, which can enter alternative biochemical pathways, depending on oxygen availability. Aerobic (¯ r-¯ ′bik) respiration occurs when oxygen is a o available. Pyruvic acid molecules enter mitochondria and, through a series of chemical reactions, called the citric acid cycle and the electron-transport chain, are converted to carbon dioxide and water. Aerobic respiration can produce 36

to 38 ATP molecules from each glucose molecule. Aerobic respiration requires oxygen because the last reaction in the series is the combination of oxygen with hydrogen to form water. If this reaction does not take place, the reactions immediately preceding it do not occur either. This explains why breathing oxygen is necessary for animal life: without oxygen, aerobic respiration is inhibited, and the cells do not produce enough ATP to sustain life. During aerobic respiration,

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Protein Synthesis

Figure 3.14

Secondary Active Transport

the carbon atoms of food molecules are broken down to carbon dioxide. Thus, the carbon dioxide humans breathe out comes from the food they eat. Anaerobic respiration occurs without oxygen and includes the conversion of pyruvic acid to lactic acid. There is a net production of two ATP molecules for each glucose molecule. Anaerobic respiration does not produce as much ATP as aerobic respiration, but it allows cells to function for short periods when oxygen levels are too low for aerobic respiration to provide all the needed ATP. For example, during intense exercise, when aerobic respiration has depleted the oxygen supply, anaerobic respiration can provide additional ATP.

Protein Synthesis
DNA contains the information that directs protein synthesis. The proteins produced in a cell are structural components inside the cell, structural proteins secreted to the outside of the cell, and enzymes that regulate chemical reactions in the cell. DNA influences the structural and functional characteristics of the entire organism because it directs protein synthesis. Whether an individual has blue eyes, brown hair, or other inherited traits is determined ultimately by DNA. A DNA molecule consists of nucleotides joined together to form two nucleotide strands (see figure 2.17). The two strands are connected and resemble a ladder that is twisted around its long axis. The nucleotides function as chemical “letters” that form chemical “words.” A gene is a sequence of nucleotides (making a word) providing a chemical set of instructions for making a specific protein. Each DNA molecule contains many different genes. Recall from chapter 2 that proteins consist of amino acids. The unique structural and functional characteristics of different proteins are determined by the kinds, numbers, and arrangement of their amino acids. The nucleotide sequence of a gene determines the amino acid sequence of a specific protein. Figure 3.15 Phagocytosis
Cell processes extend from the cell and surround the particle to be taken into the cell by phagocytosis. The cell processes surround the particle and fuse to form a vesicle that contains the particle. The vesicle then is internalized within the cell.

DNA directs the production of proteins in two steps— transcription and translation—which can be illustrated with an analogy. Suppose a chef wants a recipe that is found only in a reference book in the library. Because the book cannot be checked out, the chef makes a copy, or transcription, of the recipe. Later, in the kitchen the information contained in the copied recipe is used to prepare a meal. The changing of something from one form to another (from recipe to meal) is called translation. In terms of this analogy, DNA (the reference book) contains many genes (recipes) for making different proteins (meals). DNA, however, is too large a molecule to pass through the nuclear pores to go to the ribosomes (kitchen) where the

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(a)

(b)

Figure 3.16 Exocytosis
(a) Secretory products accumulate within vesicles whose membranes fuse with the cell membrane, releasing the contents of the vesicles to the cell surface. (b) Electron micrograph of exocytosis.

copy is used to construct a protein by means of translation. Of course, the actual ingredients are needed to turn a recipe into a meal. The ingredients necessary to synthesize a protein are amino acids. Specialized molecules, called transfer RNA (tRNA), carry the amino acids to the ribosome (figure 3.18). In summary, the synthesis of proteins involves transcription—making a copy of part of the information in DNA (a gene), and translation—converting that copied information into a protein. The details of transcription and translation are considered next.

Transcription
The events leading to protein synthesis begin in the nucleus. DNA determines the structure of mRNA through transcription. The double strands of a DNA segment separate, and DNA nucleotides pair with RNA nucleotides (figure 3.19). Each nucleotide of DNA contains one of the following organic bases: thymine, adenine, cytosine, or guanine; and each nucleotide of mRNA contains uracil, adenine, cytosine, or guanine. The number and sequence of nucleotides in the DNA determine the number and sequence of nucleotides in the mRNA because DNA nucleotides only pair with specific RNA nucleotides: DNA’s thymine with RNA’s adenine, DNA’s adenine with RNA’s uracil, DNA’s cytosine with RNA’s guanine, and DNA’s guanine with RNA’s cytosine. After the DNA nucleotides pair up with the RNA nucleotides, an enzyme catalyzes reactions that form chemical bonds between the RNA nucleotides to form a long mRNA segment. Once the mRNA segment has been transcribed, portions of the mRNA molecule can be removed, or two or more mRNA molecules can be combined.

Figure 3.17

Overview of Cell Metabolism

Aerobic respiration requires oxygen and produces more ATP per glucose molecule than does anaerobic metabolism.

proteins (the meal) are prepared. Just as the reference book stays in the library, DNA remains in the nucleus. Through transcription therefore the cell makes a copy of the information in DNA necessary to make a particular protein. The copy, which is called messenger RNA (mRNA), travels from the nucleus to the ribosomes in the cytoplasm, where the information in the

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Figure 3.18

Overview of Protein Synthesis

Translation
Translation, the synthesis of proteins based on the information in mRNA, occurs at ribosomes. The mRNA molecules produced by transcription pass through the nuclear pores to the ribosomes. The information in mRNA is carried in groups of three nucleotides called codons, which code for specific amino acids. For example, the nucleotide sequence uracil, cytosine, and adenine (UCA) of mRNA codes for the amino acid serine. There are 64 possible mRNA codons, but only 20 amino acids are in proteins. As a result, more than one codon can code for the same amino acid. For example, CGA, CGG, CGT, and CGC code for the amino acid alanine, and UUU and UAC code for phenylalanine. Some codons do not code for amino acids but perform other functions. For example, UAA acts as a signal for stopping the production of a protein. Protein synthesis requires two types of RNA in addition to mRNA: tRNA and ribosomal RNA (rRNA). There is one type of tRNA for each mRNA codon. A series of three nucleotides of each tRNA molecule, the anticodon, pairs with the codon of the mRNA. Another part of each tRNA molecule binds to a specific amino acid. For example, the tRNA that pairs with the

UUU codon of mRNA has the anticodon AAA and binds only to the amino acid phenylalanine. The ribosomes, which consist of ribosomal RNA and proteins, align mRNA with tRNA molecules so that the anticodons of tRNAs pair with the codons of mRNA while the mRNA is attached to a ribosome (figure 3.20). The amino acids bound to the tRNAs are then joined to one another by an enzyme associated with the ribosome. The enzyme causes the formation of a chemical bond, called a peptide bond, between the adjacent amino acids to form a polypeptide chain, consisting of many amino acids bound together by peptide bonds. The polypeptide chain then becomes folded to form the three-dimensional structure of the protein molecule. A protein can consist of a single polypeptide chain or two or more polypeptide chains that are joined after each chain is produced on separate ribosomes.

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Explain how changing one nucleotide within a DNA molecule of a cell could change the structure of a protein produced by the cell. Answer on page 00

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for sex cells, contains 46 chromosomes. Sex cells have half the number of chromosomes as other cells (see the section on Meiosis on p. 61). The 46 chromosomes are called a diploid (dip′loyd) number of chromosomes and are organized to form 23 pairs of chromosomes. Of the 23 pairs, one pair is the sex chromosomes, which consist of two X chromosomes if the person is a female or an X chromosome and a Y chromosome if the person is a male. The remaining 22 pairs of chromosomes are called autosomes (aw′t¯ -s¯ mz). o o The combination of sex chromosomes determines the individual’s sex, and the autosomes determine most other characteristics.

Mitosis
All cells of the body, except those that give rise to sex cells, divide by mitosis (m¯ o′sis). Mitosis involves two steps: ı-t¯ (1) the genetic material within a cell is replicated, or duplicated, and (2) the cell divides to form two daughter cells with the same amount and type of DNA as the parent cell. Because DNA determines the structure and function of cells, the daughter cells, which have the same DNA as the parent cell, can have the same structure and perform the same functions as the parent cell. The period between active cell divisions is called interphase, during which DNA is replicated. The two strands of DNA separate from each other, and each strand serves as a template for the production of a new strand of DNA (figure 3.21). Nucleotides found in the DNA of a template strand pair with nucleotides that are subsequently joined by enzymes to form a new strand of DNA. The sequence of nucleotides in the DNA template determines the sequence of nucleotides in the new strand of DNA because adenine pairs with thymine, and cytosine pairs with guanine. The new strand of DNA combines with the template strand to form a double strand of DNA. At the end of interphase, each cell has two complete sets of genetic material. The DNA is dispersed throughout the nucleus as thin threads called chromatin (kr¯ ′m˘ -tin) o a (figure 3.22a). Mitosis follows interphase. For convenience, mitosis is divided into four stages. Although each stage represents major events, the process of mitosis is continuous. Learning each of the stages is helpful, but the most important concept to understand is how each of the two cells produced by mitosis obtains the same number and type of chromosomes as the parent cell. There are four stages in mitosis: 1. Prophase. During prophase (figure 3.22b), the chromatin condenses to form visible chromosomes. After interphase, each chromosome is made up of two separate but genetically identical strands of chromatin called chromatids (kr¯ ′m˘ -tidz), which are linked at o a one point by a specialized region called the centromere (sen′tr o-m¯ r). Replication of the genetic ¯ e material during interphase results in the two identical

Figure 3.19

Transcription

Formation of mRNA by transcription of DNA chains in the cell nucleus. A segment of the DNA chain is opened, and RNA polymerase (an enzyme) assembles nucleotides into mRNA according to the base pair combinations shown in the inset. Thus the sequence of nucleotides in DNA determines the sequence of nucleotides in mRNA. As nucleotides are added, an mRNA chain is formed.

Cell Division
Cell division is the formation of two daughter cells from a single parent cell. The new cells necessary for growth and tissue repair are formed through mitosis, and the sex cells necessary for reproduction are formed through meiosis. During mitosis and meiosis the DNA within the parent cell is distributed to the daughter cells. The DNA is found within chromosomes. Each cell of the human body, except

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Cell Division

Figure 3.20 Translation of mRNA to Produce a Protein

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chromatids of each chromosome. Also during prophase, microtubules called spindle fibers extend from the centrioles (sen′tr e-¯ lz) to the centromeres ¯ o (see figure 3.1 and 3.22b). Centrioles are small organelles that divide and migrate to each pole of the cell. In late prophase, the nucleolus and nuclear envelope disappear. 2. Metaphase. In metaphase (figure 3.22c), the chromosomes align near the center of the cell. 3. Anaphase. At the beginning of anaphase (figure 3.22d), the centromeres separate. When this happens, each chromatid is then referred to as a chromosome. Thus, when the centromeres divide, the chromosome number doubles to form two identical sets of 46 chromosomes.

Each of the two sets of 46 chromosomes is moved by the spindle fibers toward the centriole at one of the poles of the cell. At the end of anaphase, each set of chromosomes has reached an opposite pole of the cell, and the cytoplasm begins to divide. 4. Telophase. During telophase (figure 3.22e), the chromosomes in each of the daughter cells become organized to form two separate nuclei. The chromosomes begin to unravel and resemble the genetic material during interphase. Following telophase, the cytoplasm of the two cells completes division, and two separate daughter cells are produced (figure 3.22f ).

Meiosis
The formation of all body cells, except for sex cells, occurs by mitosis. Sex cells are formed by meiosis (m¯ o′sis), a process in which the ı-¯ nucleus of a sex cell precursor cell undergoes two divisions, resulting in (1) four nuclei, each containing half as many chromosomes as the parent cell and (2) one chromosome from each of the chromosome pairs. The daughter cells that are produced differentiate into gametes (gam′ etz), or sex cells. The sex cells are ¯ sperm cells in males and oocytes (¯ ′¯ -s¯ tz) in o o ı females (see chapter 19). Each gamete has a haploid (hap′loyd) number of chromosomes, which is half the number of chromosomes found in other body cells. The haploid number of chromosomes in humans is 23 chromosomes. Sperm cells have 22 autosomal chromosomes and either an X or Y chromosome, and oocytes contain 22 autosomal chromosomes and an X chromosome. During fertilization, when a sperm cell fuses with an oocyte, the normal number of 46 chromosomes, in 23 pairs, is reestablished. Meiosis involves two divisions. The first division during meiosis is divided into four stages: prophase I, metaphase I, anaphase I, and telophase I (figure 3.23). As in prophase of mitosis, during prophase I of meiosis the nuclear envelope degenerates, spindle fibers form, and the already duplicated chromosomes become visible. Each chromosome consists of two chromatids joined by a centromere. In prophase I, however, the members of each pair of chromosomes lie close together. Because each chromosome consists of two chromatids, the four chromatids of a chromosome pair is called a tetrad. In metaphase I the tetrads align near the center of the cell, and in anaphase I each pair of chromosomes separates and moves toward opposite poles of the cell. For each pair of

Figure 3.21 Replication of DNA
The strands of DNA separate from each other, and each strand functions as a template for the production of a new strand. The base-pairing relationship between nucleotides (see inset) determines the sequence of nucleotides in the newly formed strand. Two identical molecules of DNA are produced, each with one new strand and one old, template strand of the original DNA molecule.

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Figure 3.22

Mitosis

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Figure 3.23

Meiosis

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Differentiation

chromosomes, one daughter cell receives one member of the pair, and the other daughter cell receives the other member. Thus each daughter cell has 23 chromosomes, and each of the chromosomes is composed of two chromatids. Telophase I is similar to telophase of mitosis, producing two daughter cells. Interkinesis (in′ter-ki-n¯ ′sis) is the period of time bee tween the first and second meiotic divisions. Replication of DNA does not take place during interkinesis. The second meiotic division also has four stages: prophase II, metaphase II, anaphase II, and telophase II. These stages occur much as they do in mitosis, except that 23 chromosomes result instead of 46. The chromosomes align near the center of the cell in metaphase II, and their chromatids split apart in anaphase II. The chromatids are now called chromosomes, and each new cell receives 23 chromosomes. In addition to reducing the number of chromosomes in a cell from 46 to 23, meiosis also dramatically increases genetic diversity for two reasons: 1. Crossing over. When tetrads are formed, some of the chromatids can break apart, and part of one chromatid can be exchanged for part of another. This exchange is called crossing over. As a result, chromatids with different DNA content are formed. 2. Random distribution. For any given person, one member of each chromosome pair is derived from the person’s father, and the other member from the person’s mother. When that person produces sex cells, during metaphase of the first meiotic division, the chromosomes align randomly, and when they split apart, each daughter cell receives some of the father’s and some of the mother’s chromosomes. How many of the father’s or mother’s chromosomes each sex cell receives is determined by chance, which is called random distribution of the chromosomes. With crossing over and random distribution of chromosomes, the possible number of gametes with different genetic makeup is practically unlimited. When the different gametes of two individuals unite, it is virtually certain that the resulting genetic makeup has never before occurred and will never occur again. Table 3.3 contrasts mitosis and meiosis.

The process by which cells are developed with specialized structures and functions is called differentiation. The single cell formed during fertilization divides by mitosis to form two cells, which divide to form four cells, and so on. The cells continue to divide until there are thousands of cells, which differentiate and give rise to the different cell types. During differentiation of a cell, some portions of DNA are active, but others are inactive. The active and inactive sections of DNA differ with each cell type. The portion of DNA that is responsible for the structure and function of a bone cell is different from that responsible for the structure and function of a fat cell. Differentiation, then, results from the selective activation and inactivation of segments of DNA. The mechanisms that determine which portions of DNA are active in any one cell type are not fully understood, but the resulting differentiation produces the many cell types that function together to make a person. Eventually, as cells differentiate and mature, the rate at which they divide slows or even stops.

Did You Know?
Through the process of differentiation, cells become specialized to certain functions and are no longer capable of producing an entire organism if isolated. Over 30 years ago, however, it was demonstrated in frogs that if the nucleus is removed from a differentiated cell and is transferred to an oocyte with the nucleus removed, a complete, normal frog can develop from that oocyte. This process, called cloning, demonstrated that during differentiation, genetic information is not irrevocably lost. Because mammalian oocytes are considerably smaller than frog oocytes, cloning of mammalian cells has been technically much more difficult. Dr. Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh, Scotland, overcame those technical difficulties in 1996, when they successfully cloned the first mammal, a sheep. Since that time, several other mammalian species have been cloned.

Did You Know?
Apoptosis (ap′op-t¯ ′sis) or programmed cell death is a normal ˘ o process by which cell numbers within various tissues are adjusted and controlled. During development, extra tissue is removed by apoptosis, such as cells between the developing fingers and toes, to fine-tune the contours of the developing fetus. The number of cells in most adult tissues is maintained at a specific level. Apoptosis eliminates excess cells produced by proliferation within some adult tissues to maintain a constant number of cells within the tissue. Damaged or potentially dangerous cells, virus-infected cells, and potential cancer cells are also eliminated by apoptosis. Apoptosis is regulated by specific genes. The proteins coded for by those genes initiate events within the cell that ultimately lead to the cell’s death. As apoptosis begins, the chromatin within the nucleus condenses and fragments. This is followed by fragmentation of the nucleus and finally by death and fragmentation of the cell. The cell fragments are cleaned up by specialized cells called macrophages.

Differentiation
A new individual begins when a sperm cell and oocyte unite to form a single cell. The trillions of cells that ultimately make up the body of an adult stem from that single cell. Therefore all the cells in an individual’s body contain the same complement of DNA that encodes all of the genetic information for that individual. Not all cells look and function alike, even though the genetic information contained in them is identical. Bone cells, for example, do not look like or function as fat cells or red blood cells.

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Table 3.3 Comparison of Mitosis and Meiosis
Feature
Time of DNA replication Number of cell divisions Cells produced

Mitosis
Interphase One Two daughter cells genetically identical to the parent cell; each daughter cell has the diploid number of chromosomes.

Meiosis
Interphase Two; there is no replication of DNA between the two meiotic divisions Gametes, each different from the parent cell and each other; the gametes have the haploid number of chromosomes; in males, four gametes (sperm cells); in females, 1 gamete (oocyte) and two or three polar bodies Gametes are produced for reproduction; during fertilization the haploid number of chromosomes in each gamete unites to restore the diploid number typical of most cells; genetic variability is increased because of crossing over and random distribution of chromosomes

Function

New cells are formed during growth or tissue repair; new cells have identical DNA and can perform the same functions as the parent cells

Did You Know?
A tumor (too′m¯ r; a swelling) is any swelling that occurs within the o body, usually involving cell proliferation. A tumor can be either malignant (m˘ -lig′n˘ nt, meaning with malice or intent to cause a a harm), able to spread and become worse, or benign (b¯-n¯n′, meaning e ı kind), not inclined to spread and not likely to become worse. Cancer (kan′ser) refers to a malignant, spreading tumor and the illness that results from such a tumor. Benign tumors are usually less dangerous than malignant tumors, but they can cause problems. As a benign tumor enlarges, it can compress surrounding tissues and impair their functions. In some cases (e.g., brain tumors), the results can be death. Malignant tumors can spread by local growth and expansion or by metastasis (m˘-tas′t˘ -sis, meaning moving to another place), which e a results from tumor cells separating from the main neoplasm and being carried by the lymphatic or circulatory system to a new site, where a second tumor forms. Cancers lack the normal growth control that is exhibited by most other adult tissues. Cancer results when a cell or group of cells, for some reason, breaks away from the normal control of growth and differentiation. This breaking loose involves the genetic machinery and can be induced by viruses, environmental toxins, and other causes. The illness associated with cancer usually occurs as the tumor invades and destroys the healthy surrounding tissues, eliminating their functions. Promising anticancer therapies are being developed in which cells responsible for immune responses can be stimulated to recognize tumor cells and destroy them. A major advantage in such anticancer treatments is that the cells of the immune system can specifically attack the tumor cells and not other, healthy tissues. Other therapies currently under investigation include techniques to starve a tumor to death by cutting off its blood supply. Drugs that can inhibit blood vessel development are currently under investigation.

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Cancer cells divide continuously. The normal mechanisms that regulate whether cell division occurs or ceases do not function properly in cancer cells. Cancer cells, such as breast cancer cells, do not look like normal, mature cells. Explain. Answer on page 00

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Systems Pathology
c y s t i c f i b r o s i s
CYSTIC FIBROSIS
Tim S. is a 5-year-old white male. He is small for his age and has had frequent bouts of pulmonary infections all his life. Tim always seemed to have a “runny nose.” None of the infections were very serious, mostly just irritating. This time, however, his congestion became so extreme that he was unable to breathe and was rushed to the hospital. There, a series of tests demonstrated that Tim suffered from cystic fibrosis. Cystic fibrosis is a genetic disorder that occurs at a rate of approximately one per 2000 births and currently affects 33,000 people in the United States. It is the most common lethal genetic disorder among whites. The diagnosis is based on the existence of recurrent respiratory disease, increased sodium in the sweat, and high levels of unabsorbed fats in the stool. Approximately 98% of all cases of cystic fibrosis are diagnosed before the patient is 18 years old. At the molecular level, cystic fibrosis results from an abnormality in chloride ion channels. There are three types of cystic fibrosis: (1) In about 70% of cases, a defective channel protein fails to reach the cell membrane from its site of production inside the cell. (2) In the second group, the channel protein is incorporated into the cell membrane but fails to bind ATP. (3) In the final category, the channel protein is incorporated into the cell membrane and ATP is bound to the channel protein, but the channel does not open. The result of any of these defects is that chloride ions do not exit cells at a normal rate. Normally, as chloride ions move out of cells lining tubes, such as ducts or respiratory passages in the body, water follows by osmosis. In cystic fibrosis, chloride ions do not exit these cells at normal rates and, therefore less water moves into the tubes. With less water present, the mucus produced by cells lining those tubes is thick and cannot be readily moved over the surface of the cells by their cilia. As a result, the tubes become clogged with mucus, and much of their normal function is lost. The most critical effects of cystic fibrosis, accounting for 90% of the deaths, are on the respiratory system. Cystic fibrosis also affects the secretory cells lining ducts of the pancreas, sweat glands, and salivary glands. In normal lungs, a thin fluid layer of mucus is moved by ciliated cells. In people with cystic fibrosis, the viscous mucus resists movement by cilia and accumulates in the lung passages. The mucus accumulation obstructs the passageways and increases the likelihood of infections. This results in chronic airflow obstruction, difficulty in breathing, and recurrent respiratory infections. Chronic coughing occurs as the affected person attempts to remove the mucus. Cystic fibrosis was once fatal during early childhood, but many patients are now surviving into young adulthood because of modern medical treatment. Currently, approximately 80% of people with cystic fibrosis live past age 20. Pulmonary therapy consists of supporting and enhancing existing respiratory functions, and infections are treated with antibiotics. The buildup of thick mucus in the pancreatic and hepatic ducts blocks them so that pancreatic digestive enzymes and bile salts are prevented from reaching the small intestine. As a result, fats and fat-soluble vitamins, which require bile salts for absorption, and which cannot be adequately digested without pancreatic enzymes, are not taken up by intestinal cells in normal amounts. The patient suffers from deficiencies of vitamins A, D, E, and K, which result in conditions such as night blindness, skin disorders, rickets, and excessive bleeding. Therapy includes administering the missing vitamins to the patient and reducing dietary fat intake. Future treatments could include the development of drugs that correct or assist chloride ion transport. Alternatively, cystic fibrosis may some day be cured through gene therapy; that is, inserting a functional copy of the defective gene into the cells of people with the disease.

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Predict the effect of cystic fibrosis on the concentrations of chloride ions inside and outside the cell. In normal muscle and nerve cells at rest, many potassium ion channels are open and potassium ions tend to flow out of the cell down their concentration gradient. How is this flow of potassium ions affected in cells of people with cystic fibrosis? Answer on page 00

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Systems Interactions
System
Integumentary

Interactions
Cystic fibrosis is characterized by increased perspiration with abnormally high quantities of sodium in the sweat which can lead to decreased blood sodium levels. A number of skin rashes and other disorders can develop as a result of the abnormal perspiration. Night blindness can develop as a result of vitamin A deficiency caused by insufficient absorption of the vitamin in the digestive tract. Diabetes mellitus resulting from decreased production of the hormone insulin may develop because blockage of the pancreatic duct by mucus results in pancreatic digestive enzymes, retained within the pancreas, destroying the pancreatic tissues (pancreatic islets), which produce insulin. Fragile blood vessels can develop, resulting in excessive bleeding. Decreased blood clotting results from insufficient vitamin K absorption from the digestive tract. Erythrocyte (red blood cell) membranes become fragile because of inadequate vitamin E absorption. The respiratory passages become clogged with viscous mucus, which blocks the airways and inhibits respiration. Recurrent respiratory infections also occur. Decreased airflow into and out of the lungs results in reduced oxygen flow to the tissues. Respiratory complications account for most deaths. Pancreatic ducts and ducts from the liver and salivary glands are blocked with thick mucus. Fats and the fat-soluble vitamins, A, D, E, and K, are poorly absorbed. Deficiencies in fat-soluble vitamins result that affect many other systems. The intestine can become impacted with dehydrated stool. Gallstones can form in the gallbladder or liver ducts. Reproductive ability is greatly decreased. In 95% of males with cystic fibrosis, there is an absence of living sperm cells in the semen. Viscous secretions in the male or female reproductive tracts decrease fertility.

Nervous Endocrine

Cardiovascular

Respiratory

Digestive

Reproductive

Summary
Cell Structure and Function
• Cells are highly organized units composed of living material. • The nucleus contains genetic material, and cytoplasm is living material outside the nucleus. • Smooth ER does not have ribosomes attached and is a major site of lipid synthesis.

The Golgi Apparatus
• The Golgi apparatus is a series of closely packed membrane sacs that function to collect, modify, package, and distribute proteins and lipids produced by the ER.

Cell Membrane
• The cell membrane forms the outer boundary of the cell. It determines what enters and leaves the cell. • The cell membrane is composed of a double layer of lipid molecules in which proteins float. The proteins function as membrane channels, carrier molecules, receptor molecules, enzymes, and structural components of the membrane.

Secretory Vesicles
• Secretory vesicles are membrane-bound sacs that carry substances from the Golgi apparatus to the cell membrane, where the vesicle contents are released.

Nucleus
• The nuclear envelope consists of two separate membranes with nuclear pores. • DNA and associated proteins are found inside the nucleus as chromatin. DNA is the hereditary material of the cell and controls the activities of the cell.

Lysosomes
• Membrane-bound sacs containing enzymes are called lysosomes. Within the cell the lysosomes break down phagocytized material.

Mitochondria
• Mitochondria are the major sites of ATP production, which cells use as an energy source. Mitochondria carry out aerobic respiration (requires oxygen).

Nucleoli and Ribosomes
• Nucleoli consist of RNA and proteins and are the sites of ribosomal subunit assembly. • Ribosomes are the sites of protein synthesis.

Cytoskeleton
• The cytoskeleton supports the cytoplasm and organelles and is involved with cell movements. • The cytoskeleton is composed of microtubules, microfilaments, and intermediate filaments.

Rough and Smooth Endoplasmic Reticulum
• Rough ER is ER with ribosomes attached. It is a major site of protein synthesis.

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Summary

Cilia, Flagella, and Microvilli
• Cilia move substances over the surface of cells. • Flagella are much longer than cilia and propel sperm cells. • Microvilli increase the surface area of cells and aid in absorption.

solid material into cells by the formation of a vesicle. Pinocytosis is similar to phagocytosis, except that the material ingested is much smaller and is in solution. • Exocytosis is the secretion of materials from cells by vesicle formation.

Whole-Cell Activity
• The interactions between organelles must be considered for cell function to be fully understood.

Cell Metabolism
• Aerobic respiration requires oxygen and produces carbon dioxide, water, and 36 to 38 ATP molecules from a molecule of glucose. • Anaerobic respiration does not require oxygen and produces lactic acid and two ATP molecules from a molecule of glucose.

Movement Through the Cell Membrane
• Lipid-soluble molecules pass through the cell membrane readily by dissolving in the lipid portion of the membrane. • Small molecules can pass through membrane channels. • Large molecules that are not lipid-soluble can be transported through the membrane by carrier molecules. • Large molecules that are not lipid-soluble, particles, and cells can be transported across the membrane by vesicles.

Protein Synthesis
• Cell activity is regulated by enzymes (proteins), and DNA controls enzyme production.

Transcription
• During transcription, the sequence of nucleotides in DNA (a gene) determines the sequence of nucleotides in mRNA; the mRNA moves through the nuclear pores to ribosomes.

Diffusion
• Diffusion is the movement of a solute from an area of higher concentration to an area of lower concentration within a solvent. At equilibrium, there is a uniform distribution of molecules. • For a given distance, a concentration gradient is equal to the higher concentration minus the lower concentration of a solute in a solution.

Translation
• During translation the sequence of codons in mRNA is used at ribosomes to produce proteins. Anticodons of tRNA bind to the codons of mRNA, and the amino acids carried by tRNA are joined to form a protein.

Osmosis
• Osmosis is the diffusion of a solvent (water) across a selectively permeable membrane. • Osmotic pressure is a measure of the tendency of water to move across the selectively permeable membrane. • In a hypotonic solution, cells swell (and can undergo lysis); in an isotonic solution, cells neither swell nor shrink; and in a hypertonic solution, cells shrink and undergo crenation.

Cell Division Mitosis
• Cell division that occurs by mitosis produces new cells for growth and tissue repair. • DNA replicates during interphase, the time between cell division. • Mitosis is divided into four stages: Prophase—Each chromosome consists of two chromatids joined at the centromere. Metaphase—Chromosomes align at the center of the cell. Anaphase—Chromatids separate at the centromere and migrate to opposite poles. Telophase—The two new nuclei assume their normal structure, and cell division is completed, producing two new daughter cells.

Filtration
• Filtration is the passage of a solution through a partition in response to a pressure difference. Some materials in the solution do not pass through the partition.

Mediated Transport Mechanisms
• Mediated transport is the movement of a substance across a membrane by means of a carrier molecule. The substances transported tend to be large, water-soluble molecules. • Facilitated diffusion moves substances from a higher to a lower concentration and does not require energy in the form of ATP. • Active transport can move substances from a lower to a higher concentration and requires ATP. An exchange pump is an active transport mechanism that moves two substances in opposite directions across the cell membrane. • Secondary active-transport uses the power of one substance moving down its concentration gradient to move another substance into the cell.

Meiosis
• Meiosis results in the formation of gametes (sperm cells or oocytes). Gametes have half the number (haploid number) of chromosomes that other (diploid) body cells do. • There are two cell divisions in meiosis. Each division has four stages similar to those in mitosis. • During meiosis the processes of crossing over within tetrads and random distribution of chromosomes increase genetic variability.

Differentiation Endocytosis and Exocytosis
• Endocytosis is the movement of materials into cells by the formation of a vesicle. Phagocytosis is the movement of • Differentiation, the process by which cells develop specialized structures and functions, results from the selective activation and inactivation of DNA.

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Chapter Three

Cell Structures and Their Functions

Content Review
1. Define cytoplasm and cell organelle. 2. Describe the structure of the cell membrane. What functions does it perform? 3. Describe the structure of the nucleus and nuclear envelope. Name the organelles found in the nucleus, and give their functions. 4. Where are ribosomes assembled, and what kinds of molecules are found in them? 5. What is endoplasmic reticulum? Compare the functions of rough and smooth endoplasmic reticulum. 6. Describe the Golgi apparatus, and state its function. 7. Where are secretory vesicles produced? What are their contents, and how are they released? 8. What is the function of the lysosomes? 9. Describe the structure and function of mitochondria. 10. Name the components of the cytoskeleton, and give their functions. 11. Describe the structure and function of cilia, flagella, and microvilli. 12. How do lipid-soluble molecules, small molecules that are not lipid-soluble, and large molecules that are not lipid-soluble cross the cell membrane? 13. Define solution, solute, solvent, diffusion, and concentration gradient. 14. Define osmosis and osmotic pressure. 15. What happens to cells that are placed in isotonic solutions? In hypertonic or hypotonic solutions? What are crenation and lysis? 16. Define filtration. 17. What is mediated transport? How are facilitated diffusion and active transport similar, and how are they different? 18. How does secondary active transport work? 19. Describe phagocytosis, pinocytosis, and exocytosis. What do they accomplish? 20. Describe how proteins are synthesized and how the structure of DNA determines the structure of proteins. 21. Define autosome, sex chromosome, diploid number, and haploid number. 22. How do the sex chromosomes of males and females differ? 23. Describe what happens during interphase and each phase of mitosis. What kind of tissues undergo mitosis? 24. Describe the events of meiosis. What happens during meiosis to increase genetic variability? 25. Define differentiation. In general terms, how does differentiation occur?

Reasoning
1. Suppose that a cell has the following characteristics: many mitochondria, well-developed rough ER, well-developed Golgi apparatuses, and numerous vesicles. Predict the major function of the cell. Explain how each characteristic supports your prediction. 2. Secretory vesicles fuse with the cell membrane to release their contents to the outside of the cell. In this process the membrane of the secretory vesicle becomes part of the cell membrane. Because small pieces of membrane are continually added to the cell membrane, one would expect the cell membrane to become larger and larger as secretion continues. The cell membrane stays the same size, however. Explain how this happens. 3. The body of a male was found floating in the salt water of Grand Pacific Bay, which has a concentration that is slightly greater than body fluids. When seen during an autopsy, the cells in his lung tissues were clearly swollen. Choose the most logical conclusion. a. He probably drowned in the bay. b. He may have been murdered elsewhere. c. He did not drown. 4. Patients with kidney failure can be kept alive by dialysis, which removes toxic waste products from the blood. In a dialysis machine, blood flows past one side of a selectively permeable dialysis membrane, and dialysis fluid flows on the other side of the membrane. Small substances, such as ions, glucose, and urea, can pass through the dialysis membrane, but larger substances, such as proteins, cannot. If you wanted to use a dialysis machine to remove only the toxic waste product urea from blood, what could you use for the dialysis fluid? a. A solution that is isotonic and contains only protein b. A solution that is isotonic and contains the same concentration of substances as blood, except for having no urea in it c. Distilled water d. Blood 5. In sickle-cell anemia a protein inside red blood cells does not function normally. Consequently, the red blood cells become sickle-shaped and plug up small blood vessels. It is known that sickle-cell anemia is hereditary and results from changing one nucleotide for a different nucleotide within the gene that is responsible for producing the protein. Explain how this change results in an abnormally functioning protein.

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Answers to Predict Questions

Answers to Predict Questions
1. p. 00 (a) Cells specialized to synthesize and secrete proteins have abundant rough ER, because this is an important site of protein synthesis. Well-developed Golgi apparatuses exist to package proteins in secretory vesicles, and numerous secretory vesicles are present. (b) Cells highly specialized to actively transport substances into the cell have a large surface area exposed to the fluid from which substances are actively transported. Numerous mitochondria are present near the membrane across which active transport occurs. (c) Cells highly specialized to ingest foreign substances have numerous lysosomes in their cytoplasm and evidence of vesicles containing foreign substances. 2. p. 00 Urea is produced continually by liver cells and diffuses from the cells into the blood. If the kidneys stop eliminating urea, it begins to accumulate in the blood and in the liver cells. The urea finally reaches concentrations high enough to be toxic to cells, causing cell damage followed by cell death. 3. p. 00 Glucose transported by facilitated diffusion across the cell membrane moves from a higher to a lower concentration. If glucose molecules are converted quickly to some other molecule as they enter the cell, a large concentration difference is maintained, and thus glucose transport into the cell continues proportional to the magnitude of the concentration difference. 4. p. 00 Changing a single nucleotide within a DNA molecule, also changes the nucleotide sequence of messenger RNA produced from that segment of DNA. The change in mRNA results in a different codon, and a different amino acid is placed in the amino acid chain for which the messenger RNA codes. Because a change in the amino acid sequence of a protein can change its structure, one substitution of a nucleotide in a DNA chain can result in altered protein structure and function. 5. p. 00 Cancer cells generally appear to be undifferentiated. Instead of dividing and then undergoing differentiation, they continue to divide and do not differentiate. One measure of the severity of cancer is related to the degree of differentiation the cancer cells have undergone. Those that are more differentiated divide more slowly and are less dangerous than those that differentiate little. 6. p. 00 Chloride ions do not move in normal amounts out of the cells of people with cystic fibrosis because chloride ion channels are defective. Instead, the chloride ions tend to accumulate inside the cell. Potassium ions tend to move out of muscle and nerve cells down their concentration gradient. The positively charged potassium ions, however, are attracted by the negatively charged chloride ions accumulated inside the cell. This attraction reduces the movement of potassium ions out of the cell and causes more potassium ions to accumulate inside the cell.

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